This invention relates to the improved construction of an offshore platform of the type used in the production of offshore oil and gas wells, which platform is provided with means for maintaining it free of marine growth so as to reduce the load on the platform and to stabilize it against horizontal overturn forces due to ocean currents and wave action and to reduce inertia loads during movements caused by earthquakes.
Present day offshore platforms used in the oil and gas industry are often formed of large diameter pipe elements in the form of four or more vertical or slanting legs interconnected or reinforced by cross-bracing tubular members. Such bottom-supported platforms have been used in waters up to 1025 feet deep. The deep water platforms may have more legs which may be tapered. For example, one deepwater platform off the California coast has eight legs that are made of 72 inch diameter pipe at the ocean floor and taper upwardly to 48 inch diameter pipe at sea level. Cross-bracing members are mostly 36 or 42 inches in diameter. In addition, the platform is provided with sixty 24 inch diameter vertical pipes, risers or well conductors which are grouped near the center of the platform and through which individual wells are drilled. Further, the platform supports vertical pipe risers through which oil and gas may be separately pumped down to an ocean floor pipeline and thence to shore.
All of the pipes or tubular members associated with oil and gas drilling and/or production offshore platforms, whether of the fixed or tension leg or floating types, are subject to the accumulation of marine-fouling growth to varying degrees to a depth of sixty feet or so in sea water and in the proper environment.
The term marine-fouling as used herein is also referred to as bio-fouling, organismic encrustation, or biotic encrustation and may take the form of clumps or colonies of barnacles, mussel, rock jingle, etc. Marine-fouling grows in the form of a sheath or coating of heavy bioencrustations which add substantial mass to tubular platform or other members and increase the effective diameter or volume of the members that are subjected to wave and/or earthquake forces.
It is known to plate the bottom of wooden ship hulls with copper to prevent the growth of barnacles. Steel ship hulls have been protected for short periods by painting them with anti-fouling paints containing, for example, copper in the form of cuprous oxide as the toxic anti-fouling material. Further, U.S. Pat. No. 3,303,118 teaches employing an electrode system immersed in sea water on the metal structure to be protected. Sufficient voltage is applied to both electrode components to produce a voltage differential high enough to provide a flow of electrolytic decomposition products toxic to marine organisms, for example, the decomposition products sodium hypochlorite, chlorine and oxygen.
It is further known to pass a direct current continuously between a ship's hull and copper electrodes carried outwardly thereof so that compounds given off by the anodes will prevent the growth of molluses and weeds, as taught in British Patent Specification No. 754,812.
Additionally, it has been shown in U.S. Pat. Nos. 2,791,096 and 4,211,503 to protect the splash zone of offshore steel tubular elements by joining to a steel tubular element, and in contact therewith, a corrosion resistant, non-ferrous metal covering sheath, for example, one made of a copper-nickel or a nickel-copper alloy. These patents are concerned with preventing corrosion due to splashing sea at a point above the normal water level. None of the above patents are concerned with providing a tough, long-lasting apparatus adapted to be arranged on an offshore steel platform or on the members thereof below the water level in a manner such that the apparatus does not act as a sacrificial anode or a cathode, while preventing the accumulation of heavy marine growth on the sections of the platform protected by the apparatus.